1. Field of the Invention
This invention relates to copper alloy metal bases for electric lamps resistant to cracking in corrosive environments. More particularly, this invention relates to copper alloy metal lamp bases which are resistant to cracking in corrosive environments, and electric lamps containing same, wherein said copper alloy contains at least about 60 wt. % of Cu and less than 10 wt. % Zn, if Zn is present.
2. Background of the Disclosure
Electric lamps comprising an outer envelope of vitreous material with an electric light source accommodated in said envelope invariably have a metal base or cap provided on an end or stem portion of said envelope, with current supply conductors connected from the light source to the base. The base containing lamp is screwed, plugged, or otherwise inserted in an electric fixture for providing current to the lamp through the base. Generally, the outer envelope of the lamp is made of a vitreous material such as glass or quartz and has a stem or neck-shaped portion, which is generally a part of the lamp envelope and which generally, but not necessarily, has a diameter smaller than that of the light source containing portion of the lamp envelope. One end of the neck-shaped portion terminates in a base portion which contains the base or cap. The base or cap is at least partly metal and has an insulating portion to avoid electrical short circuits.
Such electric lamps include the well known bulb-shaped incandescent lamps, cylindrically shaped lamps, automobile or PAR lamps and various arc discharge lamps such as the high pressure sodium vapor arc discharge lamps which generally comprise a ceramic arc tube disposed within an outer vitreous envelope and metal halide arc discharge lamps wherein the arc tube is generally made of a vitreous material such as quartz or a high temperature glass disposed within an outer vitreous envelope. All of these lamps invariably have a metal end cap or base which is either screwed, cemented or otherwise connected to one end of the stem or neck-shaped portion of the lamp envelope. Some lamps are double-ended having two stems and, concomitantly, sometimes two bases. All of these various types of lamps are well known to those skilled in the art. Illustrative, but non-limiting examples of various types of metal lamp bases may be found in U.S. Pat. Nos. 3,775,634; 4,020,382; 4,044,277 and 4,496,874. More often than not, lamps currently manufactured have metal screw bases of standardized sizes for screwing into receptacles for supplying current to the light source.
Although various metals such as aluminum, zinc plated steel, brass plated steel, nickel plated steel and certain copper alloys have been suggested for use as lamp bases, relatively high zinc content copper alloys or brasses containing at least about 10% zinc have invariably been used for the manufacture of such lamp bases. These materials possess a combination of properties, including cost, electrical and thermal conductivity, formability, resistance to mechanical damage, relatively good corrosion resistance etc., which make them desireable for use in electric lamp bases. One particular copper alloy that has been widely used is a cartridge type of brass known as a C260 alloy which is nominally 70% copper and 30% zinc on a weight basis, with less than 1% of other alloying ingredients. Another brass alloy that has been used is a low brass C240 alloy which nominally contains 80% copper and 20% zinc. U.S. Pat. No. 4,496,874 discloses suitable copper alloys as including, on a weight basis, 45-67% copper, 12-45% zinc and 10-26% nickel along with the possible addition of minor amounts of various other alloying ingredients.
One of the problems encountered in the lamp industry is the cracking of such metal lamp bases both in storage and in service. Such cracking is exhibited as cracks both parallel to and transverse to the longitudinal axis of the metal base which sometimes leads to lamp failure and/or difficulty in removing the lamp from a lamp socket, luminaire or other current supply device. Such cracks can also originate at the stake holes of lamp bases that are attached to the lamp envelope by a staking method. This also represents a potential safety hazard to a user/installer. If a base breaks off in a socket, the power to the socket must be turned off before the broken base part can be removed and a new lamp installed. Various platings over the brass, such as nickel, zinc and copper have been tried and sometimes have resulted in a small increase of the base life and concomitant lamp life in such environments, but the results have not been satisfactory.
Consequently, a need exists for metal lamp bases which are resistant to natural and man-made corrosive environments and which do not exhibit cracks in service or in storage.